In electrophotographic image formation, an electrostatic latent image corresponding to an image is formed on the surface of a photoreceptor. Before forming the electrostatic latent image on the photoreceptor, it is necessary to carry out a charging processing of uniformly charging the surface of the photoreceptor. There are two kinds of methods for charging the photoreceptor, a non-contact charging method and a contact charging method.
In the non-contact charging method, a so-called corotron charger or scorotron charger is used. By corona discharge caused by the charger, electric charge is supplied over the air to the photoreceptor. This non-contact charging method is advantageous in that since the charger does not contact the photoreceptor, it is possible to reduce the contamination and abrasion of the photoreceptor. In contrast, the non-contact charging method is disadvantageous in that a by-product, such as ozone, is generated due to the corona discharge.
In recent years, in light of environmental consideration, there has been increasing interest in contact chargers which does not use the corona discharge. Among the contact chargers, there is a charger which causes a rubber member, to which a voltage is applied and which is in the shape of a roller, to contact the photoreceptor. A roller including the rubber member is generally called a charging roller.
Document 1 describes a charging roller whose surface's friction coefficient is set to 0.4 or more to prevent poor charging caused due to nonuniform contamination on the surface of the charging roller.
Moreover, Documents 2 to 9 describe a technique of carrying out a surface treatment with respect to the rubber member of the charging roller. According to the techniques described in Documents 2 to 9, by modifying (hardening) the surface of the rubber member, made of an epichlorohydrin based rubber base material, using an isocyanate compound, it is possible to prevent leakage of, for example, an ionic conductive agent from the surface of the rubber member without further forming a layer around the rubber member.
However, the charging roller having been subjected to the surface treatment described in Documents 2 to 9 has a problem in that the ionic conductive agent contained in the rubber member leaks out when the charging roller is placed under conditions of high temperature and high humidity for a long time. If the ionic conductive agent leaks out from the charging roller, the leaked-out ionic conductive agent contaminates the photoreceptor. As a result, when forming an electrostatic latent image on the surface of the photoreceptor by a light irradiation process carried out after the charging, the electric charge remains in a contaminated region, and a white patch and/or a low concentration region is formed within a formed image.
(Document 1)
Japanese Unexamined Patent Publication No. 268583/1992 (Tokukaihei 4-268583, published on Sep. 24, 1992)
(Document 2)
Japanese Unexamined Patent Publication No. 281830/1993 (Tokukaihei 5-281830, published on Oct. 29, 1993)
(Document 3)
Japanese Unexamined Patent Publication No. 2000-346051 (Tokukai 2000-346051, published on Dec. 12, 2000)
(Document 4)
Japanese Unexamined Patent Publication No. 2001-348443 (Tokukai 2001-348443, published on Dec. 18, 2001)
(Document 5)
Japanese Unexamined Patent Publication No. 2002-40760 (Tokukai 2002-40760, published on Feb. 6, 2002)
(Document 6)
Japanese Unexamined Patent Publication No. 2002-82514 (Tokukai 2002-82514, published on Mar. 22, 2002)
(Document 7)
Japanese Unexamined Patent Publication No. 2004-191960 (Tokukai 2004-191960, published on Jul. 8, 2004)
(Document 8)
Japanese Unexamined Patent Publication No. 2004-191961 (Tokukai 2004-191961, published on Jul. 8, 2004)
(Document 9)
Japanese Unexamined Patent Publication No. 2006-53544 (Tokukai 2006-53544, published on Feb. 23, 2006)